X ray imaging is a common tool used in cardiovascular angiography. Blood vessels are imaged, usually in fluoroscopic mode up to 30 frames per second, while being infused with Iodine based contrast agent. It is a highly desired purpose to increase the image contrast of the blood vessels relative to the surrounding tissues so as to facilitate the interpretation of the results. In the same time, it is highly necessary to reduce radiation dose to which the examined subject as well as personnel are exposed.
One common technique to improve visual contrast is digital subtraction angiography (DSA). In DSA, a “mask” image is acquired before infusion of Iodine contrast agent and subsequently subtracted from images with Iodine contrast. In this way, the appearance of blood vessels is amplifying. However, DSA is artifacts prone due to motion of the patient between the time the mask is acquired and the time the contrast images are acquired. Therefore, DSA is not usually used in coronary angiography as the coronary arteries are at a constant motion due to the heart activity.
Another common technique to improve image contrast and differentiate better between absorbing element composition is dual energy subtraction. According to this technique, the subject is imaged twice with two different energy spectra of the X ray source. Different energy spectra may be obtained by applying different X ray tube voltage, different tube anode material, different beam filtering or a combination of the above. A category within this technology is K edge subtraction imaging where the two energy spectra are chosen so one spectrum peaks below the K edge of a material of interest, say Iodine, and the other peaks just above the K edge. However, this technique is not applicable to cardiac angiography with conventional X ray sources as the time to alternate between two energies is relatively long and does not allow for motion artifact free image subtraction.
U.S. Pat. No. 6,356,617 discloses a device for energy subtraction cardiac angiography based on irradiating the patient with monochromatic X ray beam from synchrotron radiation at two alternating energies. However, monochromatic X ray beams with sufficient intensity are available in just a few synchrotron radiation accelerator laboratories worldwide, so this solution is not practical for routine applications.
Practically, all X ray detectors currently in use in medical imaging are based on the principle of charge integration. Detector elements convert incoming X rays to electric charge, charge from many X rays is integrated for a certain time window and the signal is digitized to provide the output signal for the detector element. This description applies to various detectors in use in medical X ray imaging systems, regardless of the technology of the detector.
Because the radiation from X ray tubes come at a wide spectrum of energies and the charge generated by each X ray is proportional to the energy transferred from the X ray to the detector, the signal in conventional detectors is proportional to the sum of the X rays times their energy rather than simply to the number of X rays. Therefore, the output signal is biased to higher energy photons. Higher energy photons carry less diagnostic information than lower energy photons, as they are less sensitive to differences between tissues in the body, so the image contrast is reduced as compared to a theoretical system that does not bias the output to higher energy photons. On the other hand, gamma cameras and PET scanners are based on single photon counting. In these devices, each photon is counted separately and the output signal is purely the number of photons detected in a detector element.
The advantage of single photon counting for X ray imaging is recognized and a number of devices have been developed and tested. The following extensive review is brought herein as reference: B. Mikulec “Development of segmented semiconductor arrays for quantum imaging” Nuclear Instruments & Methods in Phys. Research A 510 (2003) 1-23.
The devices under discussion are built of a matrix of X ray detection elements, each element connected to a separate signal processing circuitry. The electronics provides a digital count of the number of photons detected by each element in a given time frame.
It was also realized (same reference) that by segmenting the imaging data according to X ray photon energy, images of higher image contrast might be achieved. This can be achieved e.g. by setting thresholds on each photon signal level and counting separately photons at two or more energy bins.
Incorporating detection devices enabling single photon counting in an apparatus for angiographic X ray imaging, especially for coronary angiography, provides a new and improved apparatus by which motion artifacts generated from heart beats and breathing movements are substantially eliminated.